The Association of British Riding Schools has shared some really useful advice regarding horse and rider weights. Some conclusions include:
A horse should carry no more than 20% of the their own weight (eg for a 500kg thoroughbred this would be 100kg)
This includes tack do must be deducted to find a rider allowance Inexperienced or unbalanced riders should have their weight increased by 10% (eg an 80kg rider becomes 88Kg)
Weight carrying capacity should be adjusted sensibly with consideration to confirmation, body condition, age, fitness, concurrent conditions etc
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